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Fungi of the Gastro-Intestinal Tract

An Essay

The GIT of herbivores is nutrient rich and oxygen poor. This creates an unusual habitat in which we find a huge diversity of microbes. The microbes have been examined mostly in ruminants of commercial importance. The microbes include a group of anaerobic organisms tentatively placed in the Chytridiomycota that are commonly called “rumen fungi”.

Physiologically, rumen fungi are very interesting. They utilise a wide range of enzymes, among them powerful cellulases and xylanases that degrade cellulose and hemicellulose, respectively. As yet the hypothesis that lignin degradation takes place in the GIT remains unsupported. However, these data indicate that rumen fungi have an important role in degrading plant fibre during the digestion of plant material in ruminants and probably most herbivores.

Microbes contribute other nutrients to their hosts. Herbage commonly lacks vitamins and essential amino acids. Microbes release these factors either directly or during their degradation. The factors may be absorbed by their host through the wall of the GIT. The role played by fungi, specifically, in this aspect is unclear. The comparative rates of degradation of plant material by fungi alone and in combination with other microbes indicate that digestion is a synergistic process. Release of nutrients by microbes is essential for animal nutrition.

Microbes also detoxify herbage. Plant defence compounds are extremely toxic organic molecules, including alkaloids, diterpenes, and toxins of microbial origin. Microbes remove the toxic components from the food, often using them as an energy source. The resultant food is thus more palatable for the host. The role of microbes in digestion is complex.

The presence of oxygen can be extremely harmful to anaerobic organisms. The enzymes superoxide dismutase and catalases are needed to defuse the reactive nature of oxygen, and they are lacking in all anaerobes. Rumen fungi must be cultured under strictly anaerobic conditions, and the absence of the enzymes makes isolation and culture of the fungi difficult.


Diagram of a zoospore.

Microbes of the GIT differ in other ways. The anaerobic rumen fungi lack mitochondria. They have hydrogenosomes, organelles commonly assumed to provide energy in the form of ATP. LINK Hydrogenosomes are normally located close to the base of the flagella in the zoospores (shown at right), where considerable energy is used. Little is known about the efficiency of hydrogenosomes. However, energy release from glucose in hydrogenosomes is considerably less than from mitochondria. This suggests that anaerobic fungi, while ideally suited to their habitat, are unlikely to be competitive outside the GIT.

Rumen fungi are found in the GIT of all herbivorous mammals that have been examined, including the marsupials and mammals common in Australia. In the Australian mammals, fermentation may take place in the foregut or hindgut. Animals commonly lack the capacity to digest even small polypeptides and thus rely on their microbial symbionts to complete the task for them.

It is likely that similar fungi will be found in herbivores in aquatic environments, and arthropods that utilise plant material. However, some herbivorous insects appear to have very short periods for digestion, and clarification of their mechanisms of digestion is needed. The complex microbial communities of the GIT probably play an essential role in the function of herbivorous animals. LINK

Fungi of the GIT grow by hyphal (rhizoidal) extension. Filamentous growth enables colonisation of the surface and interior of plant fragments. In comparison to bacterial digestion or invagination by protists, enzymic digestion of particles by chytrids is more rapid and complete for the short period plant material is present in the GIT. The mode of growth also enables colonisation of larger fragments as soon as they enter the digestion chamber. Further physical fragmentation by the animal would result in the fungal population also fragmenting, leading to a more rapid dissolution of the plant material. While the relative importance of fungi in digestion remains conjectural, fungi appear to play an important part in digestion of cellulosic materials and of larger and more complex fragments.

Rumen fungi are assumed to disperse in dung. While the mode of transfer from host to host is unknown, contact between parent and offspring is thought to provide the initial inoculum, and that this is via faecal sources. External transmission means that the fungal propagules must be protected from atmospheric oxygen during dispersal. The mechanism associated with survival of dispersal structures is unknown.

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References

Mountford D.O. and Orpin C.G. (1994) (Eds) Anaerobic Fungi. Dekker, New York.

Klieve AV (1996) Gut-inhabiting Fungi of Australian Herbivores. In Fungi of Australia Vol 1B. Fungi in the Environment, ABRS, Canberra.

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